IT SOUNDS like the plot of an action thriller by Tom Clancy or Jack Higgins: An American businessman is recruited by the FBI and Britain’s MI5 to penetrate a maverick group of ruthless IRA guerrillas.

But this is not fiction.

For three years, David Rupert, a 49-year-old truck driver from a small town in upstate New York, was an agent at the center of a dangerous, high-stakes game of international terrorism and intrigue.

Today, he and his wife, Maureen, endure the daily tedium of FBI protective custody – in seclusion, constantly fearful, rarely alone, but safe from those who would kill him.

In a few weeks, he will be spirited to Ireland to testify at the trial of one of the IRA’s most ruthless leaders, a trial with implications for the entire peace process.

“He’s a rat bastard,” said a bitter John McDonagh, leader of one of the New York Irish Republican groups penetrated by Rupert and turned inside out. “He’s a disgusting rat and an informer. He was introduced to our group by people in Chicago. We were suspicious of him from the start.”

Rupert is an imposing, charismatic and mysterious figure, a towering 6 feet 4 inches tall and an intimidating 280 pounds. He is part Mohawk Indian and was raised in Madrid, N.Y. He is believed to have served in the Special Forces in Vietnam.

Frank O’Neill, former chairman of the Chicago branch of the Irish Freedom Committee, also penetrated by Rupert said: “He’s a big guy. The kind of man you could see coming from a mile away.”

After years drifting from job to job, Rupert traveled to Ireland and ran bars in County Leitrim and County Donegal – rural communities rife with Provisional IRA activity.

He returned to the United States to open a trucking business, driving his lone rig all over the Midwest. In 1997, he joined a group of Irish-American activists in Chicago raising money for Irish Republican causes.

IT WAS a critical time. The Provisional IRA’s leaders were involved in U.S.-sponsored peace talks. Those talks culminated in April 1998 in the Good Friday Agreement, in which the Provisional leaders like Gerry Adams and Martin McGuinness joined their implacable enemies in a new legislature.

The talks split the Provisionals and their supporters in the United States. Michael McKevitt, 51, third in command of the Provos, broke away to form the Real IRA with about 70 hard-liners. McKevitt is married to Bernadette Sands, sister of famed IRA hunger striker Bobby Sands.

In August 1998, the Real IRA carried out one of the worst atrocities of the blood-spattered Troubles: the car bomb in the Northern Ireland town of Omagh, which killed 29 people and left 220 wounded.

The renegade faction became a threat to the peace deal, and the FBI and MI5, Britain’s security service which battles espionage and terrorism, made destroying the fledgling group a priority. By this time, Rupert had become a well-known figure in Irish Freedom Committees in New York and Chicago. The group opposes the Good Friday Agreement.

Acting on his own initiative, Rupert shuttled between cities with large Irish-American communities, including San Francisco, Chicago, Boston, New York and Philadelphia, recruiting members and soliciting funds.

McDonagh told The Post: “Rupert would fly in to New York for meetings. I wondered where he got his money from. It’s not cheap to fly from Chicago. It’s difficult enough to get people to go to a meeting on the subway.

“He was constantly trying to push the group into doing things we did not want to do and which we were not set up to do. He was an agent provocateur.”

Rupert made several trips to Ireland and, posing as a tireless fund-raiser, became close to McKevitt, Bernadette Sands, and others.

At what point Rupert began working for the FBI and MI5 isn’t clear. One well-informed Irish-American source suggests Rupert was recruited in 1998 by agents using his money troubles and the legal problems of a family member as leverage.

He began wearing a wire during meetings with McKevitt and other members of the Real IRA. He was aided by his enormous bulk and oversized clothes which defied routine frisks and searches.

“It demanded courage and a cool head,” a source in Northern Ireland’s Royal Ulster Constabulary said. “If he had been exposed while still in the Irish Republic he could have expected no mercy from his so-called associates.

“A bullet in the back of the head would have been the best thing he’d have had to look forward to. Most likely his body would never have been found.

“People can call this man a rat or an informer, but to some he is a hero; someone who realized, however late, what idealism and responsibility really is.”

OVER a period of at least two years, Rupert is believed to have passed information to the FBI and MI5 about the Real IRA’s financial contributors in New York, Chicago, Boston and Rhode Island. He is also believed to have helped uncover a plot by a group in Boston to run arms to the Real IRA.

McDonagh says he uncovered e-mails from Rupert which indicate he planned to wrest control of the Irish Freedom Committee, which would have put a double agent at the center of radical Irish-American activities.

But that was superseded by the opportunity to put McKevitt himself out of action. Rupert’s handlers put his information on McKevitt and the Real IRA leadership in the hands of Irish prosecutors.

Three weeks ago, McKevitt was arrested and charged with IRA membership and “directing an unlawful organization.” The charges carry a life sentence.

MI5 offered to place Rupert in a safe house in the Caribbean where vulnerable witnesses against the IRA have been housed, but Rupert is believed to have chosen an FBI safe house in a small town in Indiana.

New York lawyer Martin Galvin, one of the city’s most prominent Irish-American activists, met Rupert while he was working undercover. He said the FBI operation should outrage Irish-Americans.

“Supergrass trials in which former activists are persuaded to betray their comrades failed miserably when the British tried to use them in the 1980s and it would be my expectation that the tactic would fail miserably once again.

“It concerns me very deeply as an Irish-American that our government is enlisting American citizens to act as spies on behalf of the British government to try to undermine the First Amendment right of Irish-Americans who believe in an end of British rule and who work towards that goal legally in the United States.”

McKevitt’s trial is expected to take place this summer. After testifying, Rupert will be given a new identity. He will be offered cosmetic surgery to change his appearance and he will try to shed at least 50 pounds. He will also be given enough money to create a new life.

But for the rest of his life he’ll look over his shoulder, literally and figuratively. He’ll always wonder if the footsteps behind him, a distant car or an unexpected parcel bring a payback from the ruthless men in Ireland he has betrayed.